1. Field of this Invention
This disclosure relates to a semiconductor memory device, and more particularly, to a flash memory device including a one-time programmable block (hereinafter, referred to as ‘OTP block’).
2. Description of Prior Art
Electrically erasable and programmable flash memory devices have been used in various applications. The flash memory device stores data requiring no security (hereinafter, referred to as public data) and includes a memory cell array of EEPROM cells. Recently, as the complexity of information process systems has increased, the users want to store device ID data, such as a serial number of the product company, a manufacture date, security-required data, and the like, in the flash memory device together with the public data. For this, a storage region (hereinafter, referred to as an OTP block) for storing security data is configured into the flash memory device. Like the storage region for storing the public data, the OTP block comprises electrically erasable and programmable cells. To prevent the security data from being damaged or outflowing, the users want to program the security data in the OTP block only one time. However, the flash memory device with the OTP block and the memory system including the same undergo some problems.
1) In the case that a program operation for the OTP block is allowed, data stored in the OTP block may be damaged by a non-permitted user if the program command is known. For example, with the characteristics of the flash memory device, a memory cell in a programmed state is not programmed while a memory cell in an erased state is programmed. Therefore, the important security data stored in the OTP block may be damaged. Once the OTP block is programmed, the program operation for the OTP block should be originally cut off.
2) To identify whether the OTP block is programmed has a difficulty of always accessing (or reading) the OTP block.
3) As well known, the EEPROM cell will be erased so as to allow reprogramming new data in the programmed EEPROM cell. Hence, it is possible to prevent the OTP block from being reprogrammed by limiting the erase operation for the OTP block. A technique for cutting off the erase operation of the OTP block is disclosed in JP laid-open No. 2001-035176. According to the JP laid-open No. 2001-035176, an erase operation for the OTP block is preventable by inserting a fuse in a row decoder that selects the OTP block, and selectively cutting the inserted fuse. However, the technique disclosed in the above patent requires additional fuses and a process for cutting the fuses.